Abhesive coatings are used on a large scale for coating sheet-like materials, in order to reduce the adhesive tendency of adhering products towards these surfaces. Abhesive coating compositions are used, for example, to coat papers or films, which serve as support for pressure-sensitive labels. The labels, provided with pressure-sensitive adhesives, adhere to the coated surface only to an extent sufficient to make it possible to handle the support films carrying the adhesive labels. It must, however, be possible to pull the labels from the coated support film without appreciable loss of adhesive force for later use. Further possibilities for employing abhesive coating compositions are packaging papers, especially for packaging adhesive goods. Such abhesive papers or films are used, for example, for packaging foods or technical products, such as bitumen.
As abhesive coating compositions, cross linkable organopolysiloxanes, especially radiation-curable organopolysiloxanes, modified with acrylate ester groups, have proven themselves.
For instance, a method is disclosed in the German Patent No. 2,948,708 for the preparation of pentaerythritol triacrylate ester- or pentaerythritol trimethacrylate ester-modified organopolysiloxanes from organochloropolysiloxanes, optionally with the addition of HCl-binding neutralizing agents, in which organopolysiloxanes of the general formula ##STR1## (R.sup.1 =alkyl with 1 to 4 carbon atoms, vinyl and/or phenyl, with the proviso that at least 90 mole percent of the R.sup.1 groups are methyl; "a" has a value of 1.8 to 2.2; "b" has a value of 0.004 to 0.5) are reacted at first with, based on the SiCl groups, at least 2 molar amounts of a dialkylamine, the alkyl groups of which in each case have 3 to 5 carbon atoms and the carbon atoms adjacent to the nitrogen carrying in each case no more than one hydrogen atom. The reaction product is then allowed to react with at least equimolar amounts of pentaerythritol triacrylate or pentaerythritol trimethacrylate, whereupon solid components are separated from the end product by known methods.
A coating composition prepared by this method already exhibits good abhesive properties, adhesive tapes in contact with the coating composition largely retaining their adhesive power towards untreated substrates. It has, however, turned out that the property of abhesiveness must always be viewed in conjunction with the chemical structure and the structure of the adhesive, towards which the coating material is to exhibit abhesive properties. The abhesive coating material described in the German Patent No. 2,948,708 was therefore unable to provide satisfactory results in all cases, since its properties could not be adapted to the different adhesives.
Improved properties are already shown by (meth)acrylate ester-modified organopolysiloxane mixtures, which are characterized by the fact that they consist of an equilibrated organopolysiloxane with, on the average, &gt;25 to &lt;200 silicon atoms, of 2 to 30% by weight of organopolysiloxanes with, on the average, 2 to 25 silicon atoms and of 2 to 30% by weight of organopolysiloxanes with, on the average, 200 to 2,000 silicon atoms. The organopolysiloxanes contained in this ternary mixture have different tasks. The low molecular weight fraction essentially has the task of bringing about the adhesion between the coating composition to the substrate. The high molecular weight fraction serves predominantly for bringing about the desired abhesiveness of the coating composition.
The middle fraction is the curable matrix, which is responsible especially for the physical properties of the coating composition. Those skilled in the art will understand that this is only a simplified description of the properties and tasks of the three different fractions, since the complex properties, which an abhesive coating agent must have, can be obtained only through the combined action of the three components. With the modified organopolysiloxane mixture of the German Patent No. 3,426,087, it has become possible, on the one hand, to improve the abhesive properties of the mixture towards adhesive surfaces and, on the other, to improve the adhesive properties towards the substrate, on which the coating material is applied and on which it is cured. Nevertheless, it has turned out that the properties of these coating compositions cannot yet be adapted adequately to the different adhesives.
In the European Offenlegungsschrift No. 0,159,683, electron beam-curable, liquid coating materials are described, which should contain:
1. 60 to 95 parts of an organopolysiloxane with more than about 25 siloxane groups per molecule and 2 to 10 parts of reacted carbonyl groups per molecule, the remaining substituents on the silicon being hydrocarbon groups with 1 to 20 carbon atoms; reacted carbinol groups are defined as esters of acrylic acid, methacrylic acid or mixtures or ethers of a hydroxyalkyl ester of these acids, the alkyl group containing 2 to 4 carbon atoms; unreacted carbinol groups should essentially not be present anymore, so that the hydroxyl number is less than 10;
2. 3 to 25 parts of a polyester of a multihydric alcohol with acrylic acid, methacrylic acid or mixtures of these acids, the multihydric alcohol containing 2 to 4 hydroxyl groups per molecule and having a molecular weight of less than 1,200;
3. 1 to 10 parts by weight of acrylic acid, methacrylic acid or mixtures of these acids.
The additional use of the (meth)acrylate ester of a polyalcohol increases the rate of curing; on the other hand, as a result of the increase in the organic portion, it has a disadvantageous effect on the flexibility and abhesiveness of the coating material. The free acrylic acid or methacrylic acid content is an additional disadvantage. This content leads to an annoying odor and, during the application on the material to be coated, makes the processing difficult.
Because they are radiation curable, organopolysiloxanes with acrylate ester groups have been suggested for a series of further possible applications. Acrylate ester-modified organopolysiloxanes are used as coating lacquers for sealing and embedding electric and electronic components and for producing shaped objects. In connection with the possible structures of such acrylate ester-modified polysiloxanes, the following Offenlegungsschriften, Auslegeschriften and patents are named:
The German Auslegeschrift No. 2,335,118 relates to optionally substituted, acrylate group-containing organopolysiloxanes of the general formula ##STR2## (R=hydrogen or monovalent hydrocarbon groups with 1 to 12 carbon atoms; R.sup.1 =monovalent, optionally halogenated hydrocarbon groups or cyanoalkyl groups with 1 to 8 carbon atoms; R"=divalent hydrocarbon groups with 1 to 18 carbon atoms or C--O--C bonds containing divalent hydrocarbon groups; R'"=R""O.sub.0.5 or R.sub.3 'SiO.sub.0.5 ; Z=OR"", R"" or OSiR.sub.3 '; R""=alkyl group with 1 to 12 carbon atoms; "a" and "b" in each case represent numbers from 1 to 20,000; c is a number from 0 to 3; e is a number from 0 to 2; at least one of the Z groups is OR"", when c=0). The siloxane polymers can be used as intermediates in the preparation of copolymers, which contain organopolysiloxane segments and find use as coating compositions. Moreover, the acrylate functional siloxane polymers can serve as sizing agents and protective coating compositions for paper and fabrics. These products are, however, unsuitable for the preparation of abhesive coating materials. The linear, diacrylate-modified polysiloxanes of the German Auslegeschrift No. 2,335,118, moreover, by definition, contain alkoxy groups, which can be split off hydrolytically and lead to further cross linking of the polysiloxanes with worsening of the elastic properties, which are important for a coating composition.
The German Offenlegungsschrift No. 3,044,237 discloses polysiloxanes with lateral acrylate ester groups, which can be prepared by reacting epoxy-functional siloxanes of a particular structure with acrylic acid. The products obtained are radiation curable. They can be used as low viscosity lacquers for application over conventional oil-based printing inks. Only with considerable limitations are the products usable as abhesive coating compositions, since there is a hydroxyl group opposite each acrylate ester group.
The U.S. Pat. No. 4,568,566 discloses curable silicone preparations, which consist of
(a) 75 to 100 mole percent of chemically bound siloxy units of the formula R.sub.3 SiO.sub.0.5, RSiO.sub.1.5 and SiO.sub.2, as well as
(b) 0 to 25 mole percent of R.sub.2 SiO units, a number of the R units having the structure ##STR3## in which R.sup.1 is hydrogen or a hydrocarbon group with 1 to 12 carbon atoms and R.sup.2 is a divalent hydrocarbon group or an oxyalkylene group. These curable preparations are used especially for coating electronic components and as coating materials for optical fibers. They are unsuitable as abhesive coating materials for sheet-like supports because their content of R.sub.2 SiO units is too low.
The object of the European Offenlegungsschrift No. 0,152,179 is a silicone preparation, which can be cured to an elastomer. This preparation consists of (a) a silicone resin with a linear structure and, on the average, at least 150 siloxane units, as well as terminally linked acrylic acid groups, the regions in between being free of acrylic acid groups, (b) at least 10% finely divided silica and (c) a photoinitiator. These materials are to be used as adhesives and as sealing compounds.
Finally, reference is made to the European Offenlegungsschrift No. 0,169,592. This European Offenlegungsschrift relates to an optical glass fiber with a synthetic coating, with a glass fiber and an enveloping coating of a synthetic rubber with a refractive index, which is higher than that of the outer layer of the glass fiber. The synthetic rubber is formed from a curable plastics composition, which has a copolymer that contains, as monomeric units, dimethylsiloxane and at least one siloxane of the group formed by methylphenylsiloxane and diphenylsiloxane. The siloxane copolymer has at least two acrylate ester group per molecule, with the characteristic that the curable synthetic composition, moreover, contains a polyurethane acrylate with an average molecular weight of more than 3,000. The polysiloxane, named in the claim, may have the following formula ##STR4## It is an essential condition that these polysiloxanes have phenyl groups linked to silicon. The phenyl group content is necessary in order to approximate the refractive index of the coating composition to that of the glass of the optical fibers. From the use of these siloxanes in combination with a polyurethane acrylate for coating optical glass fibers, it cannot be concluded that such compounds can be used as abhesive coating compositions.
In the journal "Macromolekulare Chemie" (Rapid Communication), 7, (1986), 703 to 707, the synthesis of linear methylpolysiloxanes with terminal methacrylate ester groups is described. For this synthesis, an addition reaction between .alpha.,w-hydrogendimethylpolysiloxane and allyl epoxypropyl ether is carried out in the presence of chloroplatinic acid. The diepoxide formed is subsequently reacted with methacrylic acid in the presence of chromium diisopropyl salicylate to the desired methacrylate esters. These esters may be present in two isomeric forms: ##STR5##
If (meth)acrylate esters are prepared according to methods of the state of the art starting out from epoxy-functional siloxanes, the epoxy groups are reacted with (meth)acrylic acid. By opening the epoxide ring, mono(meth)acrylate esters with one vicinal hydroxyl group are formed.